21 Complete the sentences using the verbs in brackets in the correct form: the infinitive without to or the -ing form, as in the example. Sometimes both variants are possible. We heard the birds singing. (sing) 1. He saw them … off the tram near the post office. (get) 2. I noticed him my messages. (read) 3. She felt someone… her shoulder. (touch) 4. I saw Helen … the house. (enter) 5. My father watched the workers … … the roof. (repair) 6. She noticed him … the flower. (pick) 7. Have you ever seen him ? (exercise) 8. I heard George … to help us. (promise) 9. We looked at the children … the ducks. (feed) 10. Can you hear the thunder … ? (rumble) 11. We didn't notice my brother … … into the room. (come) the guitar? (play) 12. Have you ever heard her…